Thermally activated heat pumps based on the absorption principle hold great promise for meeting the combined environmental goals of higher energy efficiency (reduced CO.sub.2 emissions) and zero ozone depletion for space conditioning applications. However, the joint achievement of high efficiency, simplicity, and low cost has proved to be elusive. The traditional single-effect cycles, although reasonably simple, are not efficient enough. The more efficient LiBr double-effect cycle requires a separate source of cooling water and cannot be used for winter heat pumping. Various other proposed high efficiency cycles suffer from one or more of various disadvantageous requirements:
high head low flow pump(s);
high head low flow throttling device(s);
flow splitters;
internal sorbent-to-sorbent heat exchange;
separate heat transfer loops with pump(s) and/or
complex valving arrangements;
high pressure liquid seals at moving joints; and
high generator temperatures.
The use of solid sorbents in single-effect intermittent cycle heat pumps or refrigerators is well known. Solid sorbent use presents the advantages that no sorbent or refrigerant pumps or valves are required (in certain configurations) and the sorbent is reasonably well localized. There are, however, many disadvantages: high latent heat of sorption causes very low coefficient of performance (COP); achieving continuous heat flow requires multiple units connected via complex valving arrangements; the heat release rate tends to be highly uneven, and that coupled with the periodic requirement to change between absorb and desorb results in substantial idle or lightly loaded periods. To compensate for the light load periods, the apparatus must be highly loaded the remainder of the time, and the highly loaded periods determine the heat exchange surface requirements. An additional disadvantage for solid absorbents is their monovariant equilibrium, i.e., pressure is solely a function of temperature, and not of refrigerant (sorbate) content. Thus each solid absorbent operates at a unique lift, and if the lift requirement changes, e.g., due to varying ambient temperature, the sorbent cannot adjust. In contrast, the solid adsorbents do not have that problem, but they have the reciprocal problem that large temperature swings are required to desorb large amounts of sorbate.
Yet another problem with historical solid sorbent heat pumps was that the characteristic extreme sorbent volume changes (shrinking and swelling) caused the sorbent bed to compact and deactivate. That problem was largely overcome by additions of various inert conductive media, especially intricate porous structure such as activated carbon. Prior art disclosures of this solution are found in U.S. Pat. Nos. 2,986,525 and 4,595,774.
For direct-fired space-conditioning applications, the most severe limitation of single-effect solid sorbent intermittent cycles is the low COP. As a result, various multi-effect cycles have been proposed. Unfortunately, they have also increased complexity, by any of several mechanisms: a) sorbate valves and/or throttles; b) sorbent-to-sorbent heat exchange through two heat exchange surfaces; c) complex heat transfer loop valving; d) excessive generator temperature; and e) multiple sorbent beds are interconnected in conjunction with more sorbate than one sorbent bed can hold, which risks liquefying one of the sorbent beds at shutdown or abnormal conditions (all the sorbate migrates to the highest affinity sorbent).
Examples of disclosures of multi-effect solid sorbent heat pumps and their attendant complexities from the above list are: U.S. Pat. No. 5,083,607 (bc); U.S. Pat. No. 5,057,132 (abe), and U.S. Pat. No. 5,025,635 (abcde).
Rotary sorption heat pumps have been proposed. By arranging a multiplicity of single-effect intermittent cycle sorption heat pumps on a rotating frame, it is possible to achieve continuous heat pumping without either sorbent valves or heat transfer valves. Examples are disclosed in U.S. Pat. Nos. 4,478,057, 4,574,874, and 4,660,629.
The "trisorption" cycle is known in the prior art, although not by that name. It is the solid sorbent analog of a well-known liquid sorbent cycle. The liquid cycle has variously been referred to as the "two-stage evaporation, two-fold refrigeration effect" cycle (K. H. Richter, "Multi-Stage Absorption Refrigeration Systems", Journal of Refrigeration, September/October 1962, pp. (105-111) or the "double-effect condensing" cycle (J. C. V. Chinnappa, "Solar Operation of Ammonia-Water Multistage Air Conditioning Cycles in the Tropics" Solar Energy, Pergamon Press, Great Britain 1974, pp. (165-170). This cycle is characterized by achieving double-effect performance (input heat produces useful refrigerant two times) without need for internal heat exchange.
Hans Stymme, "Chemical Heat Pumps", Swedish Council for Building Research, S2:1982, Stockholm, Sweden, 1982 presents an early example of applying solid sorbents in this type of cycle. The essence is that there are three sorbents of differing affinity for the sorbate, and there is a three-stage operating cycle, each stage involving a different pair of the three sorbents in both heat and mass exchange, and each stage at a different pressure.
Uwe Rockenfeller, et al., in "Complex Compound Chemical Heat Pumps", Proceedings of the 9th Industrial Energy Technology Conference, Sep. 16-18, 1987, Houston, Texas, pp. 158-164 disclose that two of the trisorption heat pumps can be operated synchronously with phase separation to achieve a nearly continuous heat duty although with substantial fluctuations, using complex switching of the heat transfer media.
M. Lebrun, P. Meyer, and B. Spinner in "Coefficients de Performance de Machines a Froid Monoetagees: 0.8 a 1.6 Selon le Procede de Gestion des Chaleurs de Reaction", Proceedings of the XVIII International Congress of Refrigeration, Aug. 10-17, 1991, Montreal, Canada, p. 567, disclose that the low affinity media can be either a solid or simply condensed phase sorbate, and that the latter generally yields lower Coefficients of Performance.
What is needed, and a primary objective of this invention, is to simultaneously achieve the efficiency of a multi-effect cycle and the simplicity of a rotary single-effect cycle. The cycle should not require any sorbate valves, any sorbent-to-sorbent heat transfer, or any heat transfer fluid valves (other than for cooling-heating changeover). The sorbate content of each individual heat pump should be limited to positively preclude liquefaction of the sorbent. Preferably there should be no circulating heat transfer fluids beyond those inherent to the desired space conditioning function.